The regulatory aspect only comes in when shares are cashed out probably. The new RedditCoin wouldn't legally be shares so it's not bound to the same regulations. Only once the Coin is cashed into a share would it transfer and get bound to the same regulatory hurdles. At least that's the way that they'll probably try to set it up to avoid those issues.
"Securities" includes more than stock--it includes debt instruments (i.e., bonds), equity instruments (i.e., stock), and derivatives of debt or equity instruments (i.e., options).
A cryptocurrency backed by shares in reddit would probably be treated as a derivative security, and would almost certainly be regulated by the Securities Exchange Commission.
Additionally--not sure what "former SEC" lawyer they talked to, but unless they structure this offering very carefully, they will run afoul of both state and federal securities registration laws. If they structure the offering to avoid those laws (which is legal, i.e., as a private or in-state offering), then the cryptocurrency would be so limited in its availability that only an extremely tiny handful of users would ever be able to own it unless Reddit goes public.
So if they give vouchers to 100 Mio users that are redeamable if the company gets public. This would work? I mean if they give 10% of the company to reddit users via vouchers and the company gets worth 100 Billion that would be 100 Dollars per person.
IANAL